title: A planar rhombic charge-separated tetrasilacyclobutadiene.
abstract: The cyclobutadiene (CBD) molecule C(4)H(4) deviates from a high-symmetry square geometry to compensate for its antiaromatic electronic structure. Here, we report a CBD silicon analog, Si(4)(EMind)(4) (1), stabilized by the bulky 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl (EMind) groups, obtained as air- and moisture-sensitive orange crystals by the reduction of (EMind)SiBr(3) with three equivalents of lithium naphthalenide. X-ray crystallography reveals a planar and rhombic structure of the Si(4) four-membered ring, with alternating pyramidal and planar configurations at the silicon atoms. The large (29)Si chemical shift differences (Δδ > 350 parts per million) in the solid-state nuclear magnetic resonance spectra suggest a contribution of an alternately charge-separated structure. The rhombic-shaped charge-separated singlet state of compound 1 thus stabilizes its cyclic 4π-electron antiaromaticity in a manner that contrasts sharply with the bond-length alternation, characterizing the rectangular distortion of carbon-based CBD.
authors.affiliation: Functional Elemento-Organic Chemistry Unit, RIKEN Advanced Science Institute, Wako, Saitama, Japan.
authors.forename: Katsunori
authors.initials: K
authors.lastname: Suzuki
authors.forename: Tsukasa
authors.initials: T
authors.lastname: Matsuo
authors.forename: Daisuke
authors.initials: D
authors.lastname: Hashizume
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Fueno
authors.forename: Kazuyoshi
authors.initials: K
authors.lastname: Tanaka
authors.forename: Kohei
authors.initials: K
authors.lastname: Tamao